The present invention relates generally to an apparatus for raising and lowering overhead doors and, in particular, to an apparatus for winding a lift cable attached to an overhead door.
Torsion spring counterbalance assemblies are commonly utilized to counterbalance the weight of an overhead door such as a garage door. Most residential and commercial overhead garage doors are of the sectional type which are suspended on overhead tracks when the door is in the up position. A typical torsion spring counterbalance assembly includes a shaft which is journalled for rotation in two or more bearing mounts which are generally attached to the wall above the door opening such that the shaft extends generally horizontally parallel with the door lintel. Cable winding drums are secured on each end of the shaft and each drum is attached to one end of an associated cable which is wound about the drum and has its opposite end secured to the bottom panel or section of the garage door. The shaft extends through the center of a coiled counterbalance torsion spring having one end secured to the shaft by a winding plug and the other end secured to the door lintel by an anchor plug. The spring is pretensioned and releasably attached to the shaft by the winding plug. The pretensioning tends to urge the drums to rotate in the take-up direction. As the door is lowered from its overhead position, its weight is increasingly shifted from the overhead tracks to the cables. This weight is counterbalanced by the torsion spring which is being wound tighter through the cable drums and the shaft.
Typically, the cable winding drums, the winding plug and the anchor plug are formed from metal castings. A weak point in the cable winding drum is the cable entry slot through which one end of the cable passes as it enters the winding groove on the exterior of the drum. The end of the cable is knotted outside the slot to anchor the cable. Thus, as the door is raised from a closed position, the whole weight of the door is transmitted through the cable to the portion of the drum where the cable slot is formed.